Characterization of attosecond XUV pulses utilizing a broadband UV~VUV pumping

Abstract

We propose a simple scheme to characterize attosecond extreme ultraviolet (XUV) pulses. A broadband ultraviolet (UV) approximately vacuum ultraviolet (VUV) pump pulse creates a coherent superposition of atomic bound states, from which photoionization takes place by the time-delayed attosecond XUV probe pulse. Information on the spectral phase of the XUV pulse can be extracted from the phase offset of the interference beating in the photoelectron spectra using a standard SPIDER (spectral phase interferometry for direct electric-field reconstruction) algorithm. We further discuss the influence of the chirp and polychromaticity of the pump pulse, and show that they do not spoil the reconstruction process. Since our scheme is applicable for various simple atoms such as H, He, and Cs, etc., and capable of characterizing attosecond XUV pulses with a pulse duration of a few hundred attoseconds or even less, it can be an alternative technique to characterize attosecond XUV pulses. Specific numerical examples are presented for the H atom utilizing the 2p and 3p states.

Reconstruction of attosecond pulses using two-color pumping

“…By measuring the phase offset of the quantum beat appearing in the energy-resolved photoelectron signal as a function of time delay between the two-color pump and XUV pulses, the spectral phase of the XUV pulse is accurately retrieved using the standard SPIDER algorithm. Compared with the VUV SPIDER [15], the two-color SPIDER has a remarkable advantage in terms of the reconstruction accuracy and the experimental feasibility due to the use of a Ti:sapphire laser. In contrast to the commonly used methods, such as FROG-CRAB and XSPIDER, which rely on SFA in one way or another, the two-color SPIDER, as well as the VUV SPIDER and Cs SPIDER, has a more transparent reconstruction process since they do not require any approximations.…”

“…In practice, the detection angle has a finite width around Θ 0 . In a recent paper we have shown that the spectral phase error introduced by the finite width of the detection around Θ 0 is rather small and can be safely neglected [15].…”

“…Note that σ xuv I represents a relative fluctuation, and, hence, would not change after the filtering. In our previous work on the VUV SPIDER [15], we examined the influence of the phase noise σ ϕ and found that it can be effectively suppressed by choosing the energy shear, ω 21 , to be ∼10% of the bandwidth of the XUV pulse. Since this argument is obviously valid for the present case as well, we now examine the influence of the intensity noise in the photoelectron signal.…”

“…Moreover the pump wavelength needed for the Cs SPIDER is 850-890 nm, which is not very convenient if one employs a Ti:sapphire laser tuned at ∼800 nm. If, instead, two electronic states with a >1 eV energy separation are coherently excited by the vacuum ultraviolet (VUV) pump pulse with a few femtosecond duration (VUV SPIDER), an XUV pulse down to ∼100 as can be characterized [15]. Still, the remaining difficulty is the need of an intense VUV pump pulse with a few femtosecond duration.…”